This invention relates to a code division multiple access (CDMA) cellular mobile communication system comprising a base station and a mobile station and, in particular, to a reverse channel interference correction system and method, when there is an interference wave on a reverse channel in a case where a channel between the base station and the mobile station is connected, in a CDMA closed loop power control for performing a closed loop control for transmission power of the mobile station on the basis of a correction value for increasing an average received power that is generated, as a reference, an average received power in a case where there is no interference wave.
In the manner known in the art, the CDMA cellular mobile communication system carries out a transmission power control on the basis of an open loop control according to Interim Standard (IS)-95 standard of Telecommunication Industry Association/Electronic Industry Association (TIA/EIA) in a case of carrying out a start of a position registration sequence for a mobile station which moves in a cell (service area) of a base station. Inasmuch as a plurality of mobile stations use the same frequency band in the CDMA cellular mobile communication system, it is necessary to increase the number of multiple-mobile stations by decreasing interference power between the mobile station mutually.
As a result, control to an appropriate transmission power value is carried out in the CDMA cellular mobile communication system. In this case, the xe2x80x9cappropriate transmission power valuexe2x80x9d is a minimum transmission power value where a data error rate is not over an allowable value when the base station receives a signal from the mobile station.
Attention will be directed to the transmission power control of the mobile station according to the open loop control on starting of channel connection between the base station and the mobile station. In the manner which later become clear, in a conventional CDMA cellular mobile communication system, the transmission power control of the mobile station according to the open loop control is defined by IS-95-A standard and compensation of the interference on a forward channel is carried out according to IS-95-B standard.
However, in the conventional CDMA cellular mobile communication system, it is difficult to obtain a received bit energy-to-interference ratio Eb/I0 (which is a ratio of desired received wave power (Eb) to interference wave power (I0) per one bit) when a lot of mobile stations communicate with the base station. In this event, its incomplete transmission power control becomes an issue. That is, due to degradation of the received bit energy-to-interference ratio Eb/I0 (degradation of channel quality when there is the interference wave on the reverse channel) in the base station, it is impossible to maintain its stable channel quality. Accordingly, it is difficult to carry out the channel connection stably and certainly and it has contributed to deterioration in its system connectivity.
As a prior art related to the open loop control of the type, Japanese Unexamined Patent Publication of Tokkai No. Hei 10-56,421 or JP-A 10-56421 is known that provides xe2x80x9cCDMA RADIO TRANSMISSION SYSTEM, TRANSMISSION POWER CONTROLLER USED FOR THE SYSTEM AND TRANSMISSION POWER CONTROL MEASUREMENT EQUIPMENT.xe2x80x9d JP-A 10-56421 realizes adaptive control of transmission power of a mobile station in response to traffic fluctuation in terms of open loop control by correcting a control error in the open loop control for transmission power control in the code division multiple access (CDMA) radio system. According to JP-A 10-56421, a transmission power controller mounted on a mobile station is provided with a calculating circuit for calculating a desired wave reception power from a received signal, a memory circuit for storing an initial relation between the received power and a transmission power setting value, a correcting circuit for correcting the relation, and a decision circuit for deciding the transmission power set value. In this case, the transmission power controller uses a control signal included in periodically in the received signal to use the relation thereby deciding a transmission power set value from the received power while correcting the relation between the received power and the transmission power set value.
In other words, the open loop control according to JP-A 10-56421 comprises the steps of measuring an average received power or the like of the desired wave from the received signal, of detecting an error between an average received power value and a target value, and of correcting the error. In addition, the transmission power control according to JP-A 10-56421 adaptively controls the transmission power of the mobile station in accordance with an amount of traffic.
In JP-A 10-56421, it is possible to carry out correction of the open loop control on the basis of the error between the target value and the average received power value of the desired wave that is measured from the received signal in the open loop control. However, JP-A 10-56421 may be disadvantageous in that it is complicated in structure because correction of the error between the target value and the average received power value is carried out using a table.
In addition, various other CDMA mobile communication systems related to the present invention are already known. By way of example, Japanese Unexamined Patent Publication of Tokkai No. Hei 5-75,484 or JP-A 5-75484 discloses a transmission power controller which is capable of controlling transmission power at a high speed to reduce interference onto another station and to reduce power consumption. According to JP-A 5-75484, the fluctuation of a reception level is detected by a reception level fluctuation detection section. When number of times of the reception level reaching a level or below within a prescribed time exceeds a reference value, a discrimination section controls switches to calculate P1=P2xe2x88x92(V2xe2x88x92Vt) at a transmission power calculation section, where P2 is a transmission power of an opposite station obtained from a demodulation output of the received signal, V2 is a reception level from a detection section and Vt is a required minimum reception level. A modulation section is modulated by a signal representing the P1 to control a power amplifier so that the transmission power is P1. The opposite station is controlled similarly and the transmission power is controlled corresponding to the reception level fluctuation at a high speed by the open loop control. When the reception level fluctuation is less than the reference value, the switches are controlled, a received addition/subtraction request value xcex1 and a preceding transmission power P1xe2x80x2 are added as P1=P1xe2x80x2+xcex1, and amplifier is controlled so that the transmission power is equal to the P1. xcex2=Vtxe2x88x92V2 is calculated by an adder section and the value is used as an adder/subtractor request value and the modulation section is controlled for the purpose of modulation transmission.
Japanese Unexamined Patent Publication of Tokkai No. Hei 9-199,963 or JP-A 9-199963 provides a transmission power control circuit which conducts open loop transmission power control and closed loop transmission power control with high accuracy. According to JP-A 9-199963, provided with a first variable gain control amplifier for varying a gain of an input signal with a control voltage, a transmission power control circuit is provided with a second variable gain control amplifier for varying a gain of an output of the first variable gain control amplifier and an automatic power control circuit for generating a control voltage to control the gain of the second variable gain control amplifier from a distributed transmission output. The automatic power control circuit is provided with at least a comparator circuit for outputting a difference between a transmission output and a reference value, a sample-and-hold circuit for sampling and holding the output of the comparator circuit. The sample-and-hold circuit is selected to be a sampling state or a holding state in response to the transmission state to apply on/off control for the automatic power control circuit. The automatic power control circuit is set when the open loop transmission power control is in use and the automatic power control circuit is cleared when the closed loop power control is in use.
Japanese Unexamined Patent Publication of Tokkai No. Hei 8-32,514 or JP-A 8-32514 discloses a transmission power control method and a transmission power controller which is capable of reducing the effect on other mobile stations by selecting an open loop control to set transmission power of a mobile station in response to a change in a desired reception power of a mobile station when the received signal power of the mobile station is suddenly increased to reduce the transmission power of the mobile station in a short time. According to JP-A 8-32514, a mobile station measures an average desired wave received signal power per one transmission power control period from a base station and calculates an average power of a desired wave received signal in the unit of the transmission power control period and in the unit of transmission power control of one preceding time or over. When a desired wave received average power difference exceeds a reference power difference, it is interpreted that the mobile station appears at a location seen through from a shadow of a building (shadowing state). In this case, the desired wave received signal power in the base station is rapidly increased. On the other hand, the interference power from other mobile station is not so much increased. Instead, interference is imposed upon other mobile station. Then the intensity of the signal send from the base station is measured by the mobile station by open loop control and the received signal is high, then the power of the signal sent to the base station is reduced.
It is therefore an object of the present invention to provide a CDMA transmission power control method and system which are capable of gently carrying out transmission power control for a mobile station on starting of call connection.
It is another object of the present invention to provide a CDMA transmission power control method and system of the type described, in which impossibility of a closed loop control never occurs when transmission power is decreased too.
It is still another object of the present invention to provide a CDMA transmission power control method and system of the type described, in which the transmission power in the mobile station does not overshoot.
It is yet another object of the present invention to provide a CDMA transmission power control method and system of the type described, in which other mobile stations are not subjected to instantaneous large interference.
It is a further object of the present invention to provide a CDMA transmission power control method and system of the type described, which is capable of preventing call disconnection and degradation of capacity of subscribers.
Other objects of this invention will become clear as the description proceeds.
According to an aspect of this invention, a method is for correcting a reverse channel interference in a code division multiple access (CDMA) open loop control for carrying out correction of the reverse channel interference on the basis of a received power of an interference wave in a case where a base station performs an open loop control of a transmission power for a mobile station. In the above-mentioned method, the base station comprises the steps of measuring received powers of a reverse received signal that correspond to states of the interference wave on a reverse channel to produce measured received power values indicative of the received powers of the reverse received signal, of calculating an average of the measured received power values to produce a calculated average value indicative of the average of the measured received power values, of calculating, on the basis of the calculated average value and a reference value, a correction value for making the mobile station increase a current transmission power so as to become larger than a previous transmission power, and of transmitting information indicative of the correction value from the base station to the mobile station. The mobile station comprises the steps of receiving the information indicative of the correction value, of determining whether or not the correction value is renewed, and of setting transmission power of the mobile station on the basis of the correction value when the correction value is renewed.